The present invention relates generally to transmission masks of the type used for laser processing of surfaces, and, more particularly, to non-contact shadow masks utilized in laser ablation processes fabricated from a thick membrane utilizing conically undercut through holes to provide the desired ablation pattern and laser energy reflective coatings in the non-patterned areas.
In the field of microelectronic materials processing the use of high power lasers to provide selective etching or micromachining of substrate materials, metal films and layers of other materials, such as polyimide, is becoming increasingly important. Of key importance to this technology is a shadow mask or projection mask to project the pattern of the microstructures onto the material to be machined. It is well-known in the art to provide masks for ion etching, ion implantation and in optical, x-ray, ion and electron beam lithography. Masks of these types are used in relatively low power applications and typically are not suitable for high power laser applications, such as excimer laser ablation.
Prior art masks presently utilized in laser etching applications typically comprise a suitable substrate having clear or transparent areas and opaque areas to define the desired etch pattern to be projected. As increasingly higher power lasers are utilized and more exact and smaller element sizes are realized, a few micrometers (um) in size, for example, mask requirements such as reduction of defects in both the clear and opaque areas become critical.
The following criteria have been identified for ideal masks to provide optimal performance during high energy laser etching, micromachining, material deposition or other surface treatment of materials.
1. The mask object plane must be extremely thin to provide a well-defined image plane together with a full depth of focus at the image plane to provide, for example, the ability to etch deep structures in the surface of a material.
2. The mask object plane must be stable in all directions; that is, no bending or warpage of the mask object plane resulting from, for example, thermal expansion or mechanical stress or vibration of the mask.
3. The mask clear or transparent areas must be defect-free.
4. The mask opaque areas must be defect-free.
5. The mask must have a versatile alignment capability; for example, mask alignment targets should be detectable at the process laser wavelength and at a convenient alignment laser wavelength, a helium-neon laser, for example.
6. The mask should be compatible with a viable inspection scheme; that is, the mask pattern should be clearly detectable with visible radiation to allow for inspection utilizing an optical microscope, for example.
7. The mask materials, both the substrate and overlaying layers, must be stable under the high pulse rate, high power radiation typically encountered during excimer laser ablation or other high-power laser processes.
Prior art "chrome on glass" masks, shown in FIG. 1a, as used, for example, for optical lithography in IC-chip fabrication consist of a thin (several hundred Angstroms) layer of chrome defining an opaque pattern on an unstructured clear glass or quartz substrate. Typically, masks of these type will exhibit defects resulting from particulates in the transparent areas or microcracks in the substrate and pinholes in the opaque chrome layer.
U.S. Pat. Nos. 4,490,210; 4,490,211 and 4,478,677, issued to Chen et al, all assigned to the instant assignee, disclose laser etching of metalized substrates and glass materials involving an intermediate step in which the material to be etched forms a reaction product resulting from exposure to a selected gas, which is then vaporized by a beam of radiation of a suitable wavelength. Many materials can be etched directly by laser energy without the need for an intermediate step creating a reaction product material. The Chen et al patents describe non-contact masks as having a transparent substrate/body of UV grade quartz with a pattern chromium film thereon. It has been found, however, that such chromium masks cannot withstand laser energy densities of the order encountered when working with excimer or other lasers having the required intensity to etch or ablate many target materials directly. While satisfying several of the above-defined mask requirements, chromium may absorb as much as half of the incident laser energy at selected wavelengths. Thus, a single excimer laser pulse may easily ablate the chromium and destroy the opaque pattern.
U.S. Pat. No. 4,923,772 issued to Kirch et al, assigned to the instant assignee and incorporated by reference as if fully set forth herein, discloses a high-energy laser mask comprised of a transparent substrate having a patterned laser-reflective metal or dielectric coating deposited thereon. Typically referred to as a "dielectric mask", a mask, shown in FIG. 1b, comprising a transparent substrate/body having the opaque pattern formed of several layers of a highly reflective, abrasive-resistant dielectric coating provides a mask able to withstand the full range of laser intensities encountered in laser etching processes. For example, Kirch et al discloses a dielectric mask comprising many layers of such dielectric coatings deposited on a substrate of UV grade synthetic fused silica which achieves greater than 99.9% reflectivity of the incident laser energy. Such a dielectric patterned mask can withstand incident energy densities up to approximately 6J/cm.sup.2. However, defects resulting from particulates in the substrate material or microcracks and pinholes in the dielectric layers may be present. As discussed by Kirch et al, relatively pure substrate material such as UV grade synthetic fused silica is required to avoid laser absorption by impurities or inclusions in the mask clear areas. However, long-term irradiation with high-power UV radiation may induce absorptions in the mask clear areas due to solarization effects. Additionally, since the dielectric materials typically reflect radiation only in a small spectral region, near the wavelength of the processing laser, it is typically transparent in the visible range, thus presenting difficulties for a conventional optical system alignment, or inspection, utilizing, for example, an optical microscope.
Thin metal sheets fabricated from materials such as molybdenum or steel, having physical through holes formed transversely through the metal sheet representing the transparent or clear areas, referred to as metal stencil masks, as shown in FIG. 1c, have also been widely used as excimer laser masks. However, at high energy densities and high pulse repetition rates, energy absorption results in excessive heating of the mask materials, resulting in distortions in the mask object plane and rapid deterioration of the mask. Additionally, since the metal stencil masks are relatively thick, on the order of 50 um or more, scattering of the laser beam from the vertical sidewalls of the through holes degrades beam focus and image quality.
U.S. Pat. No. 4,417,946, issued to Bohlen et al, assigned to the instant assignee, discloses a mask suitable for ion etching, ion implantation and x-ray, ion and electron beam lithography. Such a mask comprises one or more metal layers deposited on a highly doped semi-conductor substrate with through going apertures defining the mask pattern. In the area of the mask pattern apertures, the substrate material is relatively thin, thus minimizing scattering effects of the incident beam. The highly doped substrate material provides mechanical stability. However, as discussed herein above, the metallic layers cannot withstand the high energy densities commonly encountered in excimer laser ablation processes.
In "Mask for Excimer Laser Ablation and Method of Producing Same," IBM Technical Disclosure Bulletin, Vol. 33 No. 1A June 1990, pp. 388-390, A.C. Tam et al considers a silicon thin membrane stencil mask having transparent areas realized by suitably dimensioned apertures through the membrane. The thermal stability of the mask is increased by coating the surface of the mask facing the laser source with a multilayer reflection system and/or coating the surface of the mask facing the material to be ablated with a metallic gold layer. However, in the high power, high repetition rate environment experienced during excimer laser ablation processes, thin membranes exhibit undesirable and intolerable bending and warping resulting from heating induced by the small amount of laser radiation transmitted through the multilayer reflection system and from mechanical stress induced in the membrane material by residual stress in the multilayer reflection system. Additionally, the silicon thin membrane mask exhibits a tendency to vibrate when exposed to high pulse repetition rates. The non-stable object plane resulting from the combined effects of mask bending and vibration significantly reduces the depth of focus and may cause scattering effects, distorting the transmitted pattern and resulting in poor image quality.